Telescopes

The Society's Telescopes

Our primary, original telescope is an 8" Newtonian (reflector), and is
a very high quality instrument, especially since its primary mirror was
re-silvered early in 1999. The telescope is near St. Aubyn's School on Blundell's
Road (see the map). Our chairman, Douglas,
is pictured with it below. For details of the telescope's history, click
here.

Douglas with the Newtonian at Milestones

In late 2001 and early 2002 we obtained five new instruments (listed below)
with our New Opportunities Fund grant from the Lottery Commission. Along
with the telescopes, SCS Astro
also generously provided us with £350-worth of extras.

Reflectors

At the simplest level, all reflecting telescopes involve the use of mirrors
to focus light rays (and lenses at the eyepiece), whereas refracting telescopes
rely purely upon lenses. The basic characteristics of different optical
configurations are charted below, starting with a common one used by most
amateur astronomers, the Newtonian. (Information from Illingworth, 1994.)
The refractors follow, and then some telescope
manufacturers and suppliers.

Inventor

Isaac Newton (English) in 1670

Benefits

Eyepiece mounted on the side of the
tube makes the small-primary Newtonian convenient to use for amateurs.

Drawbacks

Eyepiece mounted on the side of the
tube makes the large-primary Newtonian inconvenient to use for
professionals!

Notes

- Based upon work by Gregory and
Zucchi
- A 45° prism can be used in place of the diagonal mirror

Inventor

Guillaume Cassegrain (French) in
1672

Benefits

Compact optics make the telescope
portable and easy to mount; it is convenient for amateurs, and quite
popular.

Drawbacks

Small field of view, primarily due
to coma.

Notes

Inventor

James Gregory (a Scottish mathematician)
in 1663

Benefits

In the 'aplanatic' Gregorian, coma
effects and spherical aberration are eliminated because it uses an ellipsoid
primary.

Drawbacks

In the 'aplanatic' Gregorian, a small
field of view (limited by astigmatism) is the result of using an ellipsoid
primary.

- The telescope is 'catadioptric':
the correcting plate eliminates spherical aberration.
- The optics provide an extremely large field of view, as illustrated
above.

Drawbacks

Notes

Inventor

Benefits

Drawbacks

Small field of view, primarily due
to coma

Notes

Any telescope in which light emerges
along the polar axis; this means direction is fixed as the observed
object follows its path across the sky. Thus, the object can be analysed
by large pieces of stationary equipment, set up at the focus.